Welcome DullBookworm, Send your SIL an email/letter with links to www.jwfacts.com, www.freedomofmind.com, and www.jehovahs-witness.net ASAP and ask her to contact you before talking with your brother and father about the links. If you both collaborate with each other, you may be able to help your brother and father to critically think for themselves. By showing your father and brother unconditional love, being patient, and venting your frustrations on JWN, you may be able to overcome the WTBTS's BITE control that victimizes your father and brother.
The following video from Steve Hassan's website may help your SIL and you to understand how important it is to understand how the WTBTS is a dangerous cult: Strategic Interactive Approach explained 2003 (1:23:23).
Peace be with you and everyone, who you love,
Robert